Earnings per share (Computation of Basic and Diluted Net Income per Share) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Earnings Per Share [Abstract]
Net income (loss)	$ (10,660)	$ 6,907	$ 10,209	$ 8,120	$ (3,606)	$ 6,588	$ 7,535	$ 21,148	$ 14,576	$ 31,665	$ (7,175)
(Income) loss attributable to noncontrolling interests									1,330	(374)	2,715
Preferred stock dividends									(28,180)	(28,180)	(16,666)
Earnings attributable to participating securities									(16,360)	(7,828)	(947)
Net loss available to common stockholders after allocation to participating securities									$ (28,634)	$ (4,717)	$ (22,073)
Weighted average shares outstanding - basic (in shares)									60,717,301	54,836,801	50,432,590
Effect of Class O units, TSR units, FFO units and options to purchase Class A common stock on an "as if" converted basis (in shares)									0	0	0
Weighted average shares outstanding - diluted (in shares)									60,717,301	54,836,801	50,432,590
Basic net loss per share (in dollars per share)	$ (0.33)	$ (0.07)	$ (0.05)	$ (0.01)	$ (0.20)	$ (0.05)	$ (0.03)	$ 0.20	$ (0.47)	$ (0.09)	$ (0.44)
Diluted net loss per share (in dollars per share)	$ (0.33)	$ (0.07)	$ (0.05)	$ (0.01)	$ (0.20)	$ (0.05)	$ (0.03)	$ 0.20	$ (0.47)	$ (0.09)	$ (0.44)